In general, in a semiconductor device in which the main current flows in the direction of the thickness of the semiconductor substrate, the breakdown voltage increases as the semiconductor substrate thickness increases to a certain thickness determined by the resistivity of the substrate; the breakdown voltage becomes substantially fixed at the certain thickness or more. On the other hand, increasing the semiconductor substrate thickness increases the on-state resistance, which increases power dissipation and deteriorates performance.
Thus, with a semiconductor device in which the main current flows in the thickness direction of the semiconductor substrate, the optimum substrate thickness is determined in consideration of the balance between performance and breakdown voltage.
Also, in semiconductor device manufacturing, there is an optimum substrate thickness for preventing problems such as cracking, breaking and bowing of the semiconductor substrate during process steps.
For example, Japanese Patent Application Laid-Open No. 8-213292 (referred to as First Patent Document) discloses a structure for reducing the weight of a semiconductor wafer, where a plurality of recesses are formed on the back to reduce the weight without lowering the mechanical strength.
Also, Japanese Patent Application Laid-Open No. 4-192474 (referred to as Second Patent Document) discloses a structure for preventing unauthorized reading of the circuit pattern provided on a semiconductor chip, where a plurality of recesses are formed on the back of the semiconductor chip so that the semiconductor chip easily breaks when subjected to unauthorized reading.
There is an optimum substrate thickness also from the viewpoint of photolithography process, since, if the semiconductor substrate is extremely thin, then existing exposure systems etc. need focal depth control during formation of a given pattern by photolithography on the semiconductor substrate, causing great inconvenience.
Thus, in semiconductor devices in which the main current flows in the thickness direction of the semiconductor substrate, determining the substrate thickness requires considering not only the performance and breakdown voltage but also the mechanical strength of the semiconductor substrate and the photolithography processing conditions. Use of epitaxial-growth substrates is proposed in order to satisfy these conditions, but thickening epitaxial-growth layers takes time and increases cost.